On the Formation of Nitrogen Oxides During the Combustion of ...

5 Results
In general, an increase of the preheating temperature of the gas atmosphere
is followed by an increase of the nitrogen oxide (NO x ) emissions. This is due
to higher effective flame temperatures. However, in the lean burning regime,
an increase of droplet pre-vaporization results in a decrease of NO x emissions
due to the reduction in droplet size and in the total number of burning
droplets acting as hot spots. For conditions close to stoichiometry, the effect
of pre-vaporization remains moderate, and the NO x emissions are almost
independent of the pre-vaporization rate Ψ. Previous studies of Baessler et
al. [31, 32], for instance, showed that a high degree of vaporization is required
to achieve a substantial NO x abatement.
In order to achieve a better understanding of the processes involved, the NO x
emissions of n-decane (C 10 H 22 ) droplets are discussed and evaluated in detail
on the basis of experimental and numerical results. The impact of the ambient
atmosphere, droplet pre-vaporization, preheating, initial droplet size, and
reaction kinetics is investigated. The respective droplet setups and the general
conditions were introduced in the above Chapters 3 and 4.
For the numerical studies presented in the following sections, the thermophysical
properties of the liquid fuel are taken from the NIST database [311]
and verified with Abramzon and Sazhin [3] and Cuoci et al. [92]. The single
droplet model, as described in Chapters 4.2 and 4.5, is employed with
the conduction limit model being used for the liquid phase [9, 236, 381–
383, 402, 403, 405]. The initial temperature of the droplet is set to T 0 = 440 K for
all simulation runs to avoid large heat losses due to heating of the liquid fuel
and, thus, the need to cope with secondary effects. This temperature is 7K
below the boiling temperature of C 10 H 22 . Moreover, the properties of the gas
phase are calculated from the input data of the chemical reaction mechanism.
The mechanism itself is a combination of the n-decane kinetics of Zhao et al.
[474] and the nitrogen oxides chemistry of Li and Williams [250]. It was tested
for different flame types in a wide range of equivalence ratios (see Chap. 2.3).
155